1. Field of the Invention
The present invention relates to a voltage-controlled oscillator including variable delay circuits and, more particularly, to a voltage-controlled oscillator including variable delay circuits which generates a precisely tunable output signal having a wide frequency range.
2. Description of the Related Art
Phase-locked loops (“PLLs”) are widely used in clocks, for synthesizing frequency components, and for demodulating and modulating signals in communication and control systems. An important component of a PLL is a voltage-controlled oscillator. A basic function of the voltage-controlled oscillator is varying frequency according to a control voltage.
FIG. 1 is a schematic circuit diagram of a voltage-controlled oscillator of the prior art. The voltage-controlled oscillator of the prior art shown in FIG. 1 includes variable delay circuits 11, 12, 13 and 14. Each variable delay circuit 11, 12, 13 and 14 is configured to receive a respective previous differential output signal pair UP/UN from an associated adjacent previous variable delay circuit as a next differential input signal pair IP/IN.
FIG. 2 is a schematic circuit diagram of an individual variable delay circuit, e.g., one of the variable delay circuits 11, 12, 13 and 14, of the voltage-controlled oscillator of the prior art in FIG. 1. FIG. 3 is a graph of control voltage versus frequency in the voltage-controlled oscillator of the prior art in FIG. 1.
The individual variable delay circuit shown in FIG. 2 includes first and second loading circuits 21 and 22, respectively, n-channel metal-oxide semiconductor (“NMOS”) transistors 23 and 24 and a current source 25. The first and second loading circuits 21 and 22 receive a supply voltage VDD and are controlled by a control signal VCON.
The NMOS transistors 23 and 24 are connected to the first and second loading circuits 21 and 22, respectively, and are gated by signals IP and IN, respectively, of the differential input signal pair IP/IN. The current source 25 includes an NMOS transistor gated by a source bias voltage VNB.
As shown in FIGS. 2 and 3, a frequency variation of the voltage-controlled oscillator corresponding to a given change of the control voltage VCON is determined by a voltage level of the source bias voltage VNB. For example, when the source bias voltage VNB is high (corresponds to line (i) of FIG. 3) a frequency variation corresponding to the given change in the control voltage VCON is large in comparison to when the source bias voltage VNB is low (corresponds to line (ii) of FIG. 3), wherein a frequency variation to the given change in the control voltage VCON is smaller.
Further, to have a wide frequency range, the voltage-controlled oscillator is required to have a large frequency variation in response to the control voltage VCON. However, since a frequency of the differential output signal pair UN and UP is sensitive to a noise of the control voltage VCON, it is difficult to precisely adjust the frequency of the differential output signal pair UP/UN. To compensate for this, a frequency variation in response to the control voltage VCON is set to be lower, and it is therefore easier to precisely adjust the frequency variation, but this reduces a frequency range of the voltage-controlled oscillator, and the reduced frequency range is insufficient.